The 6C33 was used for the power supply for the Mig's radio system and was remarkable for its power handling capabilities. The first audio amp using this tube was built by a Mr. Dakasue in 1977.The first American design to use this tube was the Atma-Sphere Novacron. IME the tubes stand a good chance of outlasting the tube sockets, which never specced for the kind of filament current seen with the 6C33. That tube was a modified version of the Sylvania 3C33 which was a dual power triode. The Russian spec sheet calls for a 750 hour life; at which time the socket was expected to be replaced with the power tube. With such a short life and with a military application, this easily explains why so many were built (although in practice they can last much longer than that).
The sockets should be replaced about every 3000 hours or so. The filament connections overheat and loosen up. It is possible to re-tighten them once, but after that they will become problematic with the tubes not lighting up. IIRC correctly they were made by the Uljanov plant, which was torn down about 20 years ago.